Pointer's value changes without modifying - c

Question

I'm studying linked list and try to implement some basic function.
My code:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define MAX_LEN_NAME 5
#define INSERT_NODE 1
#define APPEND_NODE 2
#define DEL_HEAD 3
#define DEL_TAIL 4
#define SHOW_LIST 5
struct people {
int id;
char name[MAX_LEN_NAME];
struct people *next;
};
typedef struct people people_list;
static void node_insert(people_list** head_ref, int id, const char* name);
static void node_append(people_list** head_ref, int id, const char* name);
static void node_del_head(people_list** head_ref);
static void node_del_tail(people_list** head_ref);
static void node_show(people_list** head_ref);
static void node_insert(people_list** head_ref, int id, const char* name)
{
people_list* new_node = NULL;
new_node = malloc(sizeof(people_list));
memset(new_node, 0, sizeof(people_list));
if (new_node == NULL) {
printf("Fail to allocate memory for new node\n");
exit(1);
}
new_node->id = id;
memcpy(new_node->name, name, sizeof(name));
new_node->next = *head_ref;
*head_ref = new_node;
}
static void node_append(people_list** head_ref, int id, const char* name)
{
return;
}
static void node_del_head(people_list** head_ref)
{
return;
}
static void node_del_tail(people_list** head_ref)
{
return;
}
static void node_show(people_list** head_ref)
{
people_list* current = NULL;
current = *head_ref;
if (current == NULL) {
printf("Empty list, nothing to show\n");
return;
}
printf("Elements in the list: \n");
while (current != NULL) {
printf("id = %d, name = %s\n", current->id, current->name);
current = current->next;
if (current == NULL) {
printf("This is the last element\n");
}
}
}
int main(void)
{
people_list* list = NULL;
#if 0
people_list* second = NULL;
people_list* third = NULL;
list = malloc(sizeof(people_list));
memset(list, 0, sizeof(people_list));
second = malloc(sizeof(people_list));
memset(second, 0, sizeof(people_list));
third = malloc(sizeof(people_list));
memset(third, 0, sizeof(people_list));
list->id = 1;
memcpy(list->name, "duc", sizeof("duc"));
list->next = second;
second->id = 2;
memcpy(second->name, "hy", sizeof("hy"));
second->next = third;
third->id = 3;
memcpy(third->name, "bo", sizeof("bo"));
third->next = NULL;
#endif
char id;
char name[MAX_LEN_NAME] = {0};
int option = 0;
while (1) {
printf("******************\n");
printf("1: Insert new node\n");
printf("2: Append new node\n");
printf("3: Delete head node\n");
printf("4: Delete tail node\n");
printf("5: Show list\n");
printf("Insert your choice: ");
scanf("%d", &option);
switch (option) {
case INSERT_NODE:
printf("debug: list=%08Xh\n", list);
printf("Enter id's value: ");
scanf("%d", &id);
printf("Enter name: ");
scanf("%s", name);
printf("debug: list=%08Xh\n", list);
node_insert(&list, id, name);
memset(name, 0, MAX_LEN_NAME);
break;
case APPEND_NODE:
printf("Enter id's value: ");
scanf("%d", &id);
printf("Enter name: ");
scanf("%s", name);
node_insert(&list, id, name);
memset(name, 0, MAX_LEN_NAME);
break;
case DEL_HEAD:
node_del_head(&list);
break;
case DEL_TAIL:
node_del_tail(&list);
break;
case SHOW_LIST:
node_show(&list);
break;
default:
printf ("Invalid input, closing program...\n");
exit(0);
break;
}
}
}
How I test:
Insert first node (1, jon)
Insert second node (2, may)
Show list => Only second node is printed.
I found that, the second->next does not point to the first node, it points to a NULL node. I keep debugging as below
case INSERT_NODE:
printf("debug: list=%08Xh\n", list);
printf("Enter id's value: ");
scanf("%d", &id);
printf("Enter name: ");
scanf("%s", name);
printf("debug: list=%08Xh\n", list);
node_insert(&list, id, name);
memset(name, 0, MAX_LEN_NAME);
break;
and the console log is
Insert your choice: 1
debug: list=001F29A8h => This is the address holding value of the first node
Enter id's value: 2
Enter name: may
debug: list=00000000h => The value of list is clear ???
As you can see, the value of the ponter list changes without modifying. Please help me to fix this. Thank you.

Answer 1

As pointed out in the comments, with scanf("%d", &id);, %d is expecting an int *. Change
char id;
char name[MAX_LEN_NAME] = {0};
to
int id;
char name[MAX_LEN_NAME] = {0};
Otherwise you invoke Undefined Behaviour by passing an unexpected type to scanf.
With good compiler warnings, we get the following:
warning: 'memcpy' call operates on objects of type 'const char'
while the size is based on a different type 'const char *'
[-Wsizeof-pointer-memaccess]
memcpy(new_node->name, name, sizeof(name));
When the sizeof operator is used on a pointer (in this case, a const char *) the result is the size of the pointer in bytes, not the object it points to.
Change this function call to
memcpy(new_node->name, name, sizeof new_node->name);
/* OR memcpy(new_node->name, name, MAX_LEN_NAME); */
or more appropriately, use strcpy
strcpy(new_node->name, name);
Suggest changing #define MAX_LEN_NAME 5 to something more reasonable like #define MAX_LEN_NAME 128.
At the same time, scanf("%s", name); should be changed to have a field width specifier that limits the length of the input read, to avoid buffer overflows. This should be the buffer size minus one (e.g., scanf("%127s", name);).
Additionally, the return value of scanf should not be ignored. Ensure it is the expected number of successful conversions, and otherwise handle failure. E.g.,
if (2 != scanf("%d%u", &si, &us)) {
fprintf(stderr, "Failed to read inputs.\n");
exit(EXIT_FAILURE);
}
Aside: These function calls
people_list* new_node = NULL;
new_node = malloc(sizeof(people_list));
memset(new_node, 0, sizeof(people_list));
can be simplified with calloc
people_list *new_node = calloc(1, sizeof *new_node);